Method for making vessels for improving the mechanical strength thereof

ABSTRACT

Manufacturing process of a container ( 2 ) in a mold ( 5 ) having a cavity ( 8 ) defining the final shape of the container ( 2 ), from a polymer blank ( 3 ) heated in advance, this process comprising the following operations:
         introduction of the blank ( 3 ) into the mold ( 5 ) heated to a predetermined temperature,   introduction into the blank ( 3 ) of a pressurized explosive gaseous mixture,   ignition of the gaseous mixture.

The invention concerns the manufacture of containers, by blowing orstretch-blowing from polymer blanks.

Hereinafter, the term “blank” will not only cover the notion of apreform, but also that of an intermediate container, that is to say anobject having undergone a first blowing (which can be free) and intendedto undergo a second to form the final container.

Among the polymers currently most used for manufacturing containers arethe saturated polyesters and particularly PET (poly(-ethyleneterephthalate)).

We would like to briefly point out that stretch-blowing of containersconsists of taking a polymer blank that has been pre-heated, introducingthis blank into a mold with the shape of the end container to be formed,then stretching the blank by means of stretch rod (also called a cane)and blowing pressurized gas into it (generally air) to pin the materialflat against the wall of the mold.

Remarks Concerning PET

Here, we would like to make a few remarks concerning the physical andmechanical properties of PET, so as to better grasp what follows.

The invention is certainly not limited to the PET alone; however, giventhat this material is, at present, that most currently used for thestretch-blowing of containers, it would appear to be appropriate tostudy this example carefully.

Here following, the figures between the square brackets will refer tothe documents listed in the bibliography attached to this description.

PET is a polyester obtained by polycondensation from terephtalic acidand from ethylene glycol. Its structure can be amorphous or partiallycrystalline (without exceeding 50% however). The possibility of goingfrom one phase to another depends largely on the temperature: below itsvitreous transition (T_(g)≈80° C.), the micromolecular chains are quitemobile and the material is solid, with a congealed ((solidified))microstructure; above the fusion temperature T_(f) (about 270° C.), thebonds between the chains are destroyed and the material is liquid.Between these two temperatures, the chains are mobile and theirconformation can change (See [2]).

More precisely, above T_(g) the chain movements made possible in theamorphous disordered zones permit greater and easier deformations of thematter (the Young module, rigidity characteristic, abrupt drop fromT_(g)) (See [1]).

The PET chemical formula (See [1]) is as follows:

The crystallization of polymer occurs between the vitreous transitiontemperature T_(g) and the fusion temperature T_(f). If the polymer issubjected to a deformation or a flowing, the kinetic of crystallizationis accelerated. It is important to note that the crystalline textureobtained in this case is complex and anisotropic (See. [3]).

The energy contribution necessary to modify the structure of themacromolecular chains can be thermal (which is then referred to asnatural or static crystallization), or mechanical, by permanentdeformation of the material. This energy contribution by deformationexhibits the following advantages, among others:

-   -   It is fast (it induces the chains to stretch and to orientate,        facilitating their transition from a disordered amorphous state        to a crystallized structure),    -   the induced orientation increases the mechanical resistance of        the material,    -   the crystals formed retain the translucidity of the material,        contrary to crystals generated by thermal crystallization (the        lamellas formed remaining small in size in relation to the wave        lengths or the visible spectrum)

In this crystallized and orientated state, the PET exhibit numerousqualities: very good mechanical properties (high rigidity, goodresistance to traction and to tearing), good optical properties andbarrier properties to CO₂. (See [2])

The mechanical properties of PET are essentially a function of thecrystalline texture, the crystalline volumic fraction and the moleculardimension and orientation. It is known that these parameters areparticularly affected by the thermal history of the material (See.[4]).

The long-term structural hardening of the polymer is exclusivelyassociated with the crystallization; however, recent studies show thatthis hardening appears even without complete crystallization and can beattributed to the orientation of the macronuclear chains and theirorganization (See [5]).

These mechanical properties justify the use of PET in thestretch-blowing of containers, and particularly bottles. Stretch-blowingcauses a bi-orientation of the polymer, that is, on the one hand, anaxial orientation of the macromolecules at the time of the stretching bymeans of a stretch rod and, on the other hand, a radial orientation ofthe macromolecules at the time of the blowing.

More precisely, the stretching of the PET causes a warped or trans typechange of conformation of the molecular chains, leading to a partialcrystallization of the polymer. In microstructure terms, the benzeniccores tend to orient in a parallel plane to the main directions of thestretching. As we remarked above, the PET does not crystallize 100%; themaximum rate noted being about 50%. The containers manufactured withinthe industry, and particularly bottles, generally exhibit a rate closeto 35%. (See [2]).

We will see here following, for certain particular operatingapplications, the rate of crystallinity can be increased (up to 40% and,some assert, even more than that (See [6])).

Several known methods make it possible to measure the crystallinity rateof a polymer. The two most prevalent methods are densimetry anddifferential calorimetric analysis (better known by its English acronymDSC—Differential Scanning Calorimetry). These methods are described in[2] and summarized here following.

Densimetry is based on the determination of the density of the material:when the material crystallizes, its density increases due to thecompacter organization of the chains in the crystalline phase. Assumingthat the specific volumes of the two phases follow a mixture law, onecan then calculate the crystallinity rate by the following relationship:

$X_{c} = {\frac{d}{d_{c}}\frac{d_{a} - d}{d_{a} - d_{c}}}$

The density d of the sample is measured by successive weighings in theair and in water. The density d_(a)=1,333 g/cm³ of the amorphous phaseis a relatively well-established value. The density d_(c) of thecrystalline phase varies between 1.423 et 1.433 g/cm³ for an orientedPET having undergone a tempering between 60° C. and 100° C. Thegenerally permissible value is 1.455 g/cm³.

The DSC analysis itself consists of establishing a thermogram for theavailable polymer sample. Traditionally, this thermogram was traced byimplementing a heating of the material at a speed of 10° C./min.

An example of a thermogram is reproduced in FIG. 1, for an industrialPET (PET 9921W EASTMAN), generally used for the stretch-blowing ofbottles (See [3]) and the molecular mass of which is M_(n)=26 kg/mol.The development of the vitreous transition temperature (to T_(g)=78° C.)results in an inflexion of the curve. One then observes a firstexothermic peak centered around 135° C., characteristic of thecrystallization of the initially amorphous material (See [2]), then asecond peak, endothermic this time, at T_(f)=250° C., corresponding tothe fusion of the previously formed crystals.

The calculation of the initial crystallinity rate can be done bycomparing the enthalpy difference (area under the peaks) between fusionand crystallization, that is noted ΔH, with the fusion enthalpy H_(ref)of a PET assumed to be perfectly crystalline, the value of which isgenerally chosen at around 100 J.g⁻¹. The crystallinity rate is given bythe following equation:

$X_{C} = \frac{\Delta \; H}{H_{ref}}$

DSC analysis appears to be frequently preferred to densimetry, which isreputed to be less precise.

Context of the Invention

Having made these remarks, we will now focus on the context in which theinvention was realized.

As we have seen, the crystallinity and the molecular orientation of thepolymer have an affect on the mechanical properties. Manufacturers havelong applied this knowledge in order to increase the rigidity ofcontainers, for example in order to enable them to sustain significantpressures, possibly reaching several bars (in the case of carbonateddrinks).

Numerous manufacturers have likewise sought to apply the supposedconsequences of the crystallization of materials to the mechanicalproperties of the containers manufactured from these materials. Inparticular, it is universally asserted in scientific and technicalliterature that increasing the crystallinity reduces the shrinkage ofthe container during the hot filling of it, that is to say at atemperature greater than the vitreous transition temperature T_(g). (Theshrinkage is the result of the release of internal stresses accumulatedby the material at the time of its macromolecular orientation during thestretch-blowing (See [7]).)

Increased crystallinity has customarily been obtained by a process knownas heat setting, which consists, at the end of the blowing, of keepingthe formed container against the wall of the mold, which is heated to apreset temperature that can range up to 250° C. The container is thuskept pinned flat against the wall of the mold for several seconds. Thereare numerous recommendations, varying from manufacturer to manufacturerboth with respect to the temperature and the duration of the heatsetting. [8] and [9], specifically, propose a range of temperaturesbetween 130° C. and 250° C., and time (6 sec, 30 sec and 120 sec),specifically intended to reduce the shrinkage of the container during ahot filling. The containers having undergone heat setting to make themresistant to deformation at the time of a hot-filling are, in currentmanufacturing language, called HR (heat resistant).

[6] proposes to circulate in the container, at the end of the blowing, agas (of air) at a so-called high temperature between 200° C. and 400°C., so as to bring the inside wall of the container to a temperature ofat least 120° C. in order to increase its crystallinity. It presumed inthis document that the total duration of the manufacturing of thecontainer can be less than 6 sec, while the crystallinity rate obtainedvaries from 34.4% to 46.7%. (It should be noted that it concerns averagecontainer crystallinity rates, measured by a densimetric method akin tothat presented above.) According to [6], a crystallinity rate greaterthan 30% must be considered to be characteristic of a highcrystallinity.

It has likewise been proposed (See [10]) to implement the blowing of thecontainer in two stages: a first one during which an intermediatecontainer of greater volume than the container to be obtained is formed,then a second during which the container is molded to its finaldimensions. Between these two stages, the intermediate container isheated at a temperature between 180° C. and 220° C. for a period between1 min et 15 min. According to [10], the variation of the volume of thecontainer during its hot filling (i.e. at a temperature from 93° C. to95° C.) is less than 5%, in these conditions. One proposed explanationis that the two successive molding operations provide two crystallinebioorientations, instead of one alone in the classical processes.

In addition, it has been proposed (See [11]) to produce containers witha crystallinity on the inside greater than the crystallinity on theoutside, in order to minimize the dispersion of the aromas of theliquid.

An analysis of the existing manufacturing techniques, and particularlythose that have just be briefly remarked upon, demonstrate that anessential concern, in the proposed solutions, is to find the maximizercrystallinity for the final container, so as to reduce its shrinkage asmuch as possible when it is hot filled. Manufacturers run up against arecurrent problem: the contradiction between the crystallinity rate(which they want to maximize, as we have just seen) and the cycle time(which they want to minimize). It is in fact currently accepted that thecrystallization of the polymer becomes easier as the deformation speeddecreases (See [1] in particular). The cycle times presented in thecited documents are relatively long (greater 5 sec., some exceeding oneminute) and necessitating multiple machines to meet current requirementsin terms of rates (up to 50,000 containers per hour), which puts aburden on production costs.

Presentation of the Invention

The invention seeks to propose an alternative solution for themanufacturing of polymer containers, which specifically makes itpossible to obtain good performance when they are hot filled.

For this purpose, the invention proposes, according to a first aspect, amanufacturing process for a container in a mold having a cavity definingthe final form of the container, from a polymer blank heated beforehand;this process comprises the following operations:

-   -   introduction of the blank into the mold heated to a        predetermined temperature,    -   introduction into the blank of a pressurized explosive gaseous        mixture,    -   ignition of the gaseous mixture.

Measurements have demonstrated that a container obtained by this processexhibits a negative gradient of the inside wall of the container.

Hence, surprisingly, the inventors noted that the container obtained bythis process exhibits, with hot filling, performances at leastequivalent to those of the classical processes utilizing heat setting ofthe container. Under certain operating conditions the performances areeven better; the shrinkage of the container being very low (less than1%, on average). This is clearly in contradiction of the widespread idea(we remarked on in the introduction) that it is necessary to maximizethe crystallinity rate of the container in order to increase itsmechanical stability (that is to say, in practice, to reduce itsshrinkage) at the time of its hot filling.

The following are provided, furthermore, depending on the method ofexecution:

-   -   a pre-blowing operation during the course of which the blank is        stretched, while introducing the explosive gaseous mixture into        it;    -   an ignition operation consistent with igniting the explosive        gaseous mixture;    -   a degassing operation during the course of which the inside of        the container is brought to a pressure close to atmospheric        pressure.

As a variant, the process includes:

-   -   a pre-blowing operation during the course of which the blank is        stretched, while introducing the explosive gaseous mixture into        it;    -   an ignition operation consistent with igniting the explosive        gaseous mixture;    -   a blowing operation consistent with introducing high pressure        air into the container;    -   a degassing operation during the course of which the inside of        the container is brought to a pressure close to atmospheric        pressure.

Following the ignition operation, the process can include astabilization operation, during which the residual gas deriving from theignition is maintained in the container.

Moreover, a sweeping operation is provided prior to the degassingoperation during which air is circulated in the container.

According to another embodiment, the process comprises:

-   -   a pre-blowing operation, during the course of which the blank is        stretched, while introducing the explosive gaseous mixture into        it;    -   a primary ignition operation consistent with igniting the        explosive gaseous mixture;    -   a primary degassing operation during the course of which the        inside of the container is brought to a pressure close to the        atmospheric pressure,    -   a secondary pre-blowing operation during the course of which an        explosive gaseous mixture is introduced into the container;    -   a secondary ignition operation consistent with igniting the        explosive gaseous mixture;    -   possibly a stabilization operation, during which the residual        gas deriving from the ignition is maintained in the formed        container.    -   possibly a blowing operation consistent with introducing high        pressure air into the container;    -   possibly a sweeping operation during which air is circulated        into the container; and    -   a secondary degassing operation during the course of which the        inside of the container is brought to a pressure close to        atmospheric pressure.

According to yet another embodiment, the process comprises:

-   -   a pre-blowing operation, during the course of which the blank is        stretched, while introducing the pressurized air into it;    -   a blowing operation consistent with introducing high pressure        air into the container;    -   a primary degassing operation during the course of which the        inside of the container is brought to a pressure close to the        atmospheric pressure,    -   a secondary pre-blowing operation during the course of which an        explosive gaseous mixture is introduced into the container;    -   an ignition operation consistent with igniting the explosive        gaseous mixture;    -   possibly a stabilization operation, during which the residual        gas deriving from the ignition is maintained in the formed        container;    -   possibly a sweeping operation during which air is circulated        into the container.    -   a secondary degassing operation during the course of which the        inside of the container is brought to a pressure close to        atmospheric pressure.

The mold is preferably heated to a temperature greater than or equal to100° C. This temperature is about 130° C. according to the method ofexecution. As a variant, this temperature is about 160° C. environ.

The explosive gaseous mixture can comprise air and hydrogen, for examplewith a volumetric proportion of hydrogen of about 6%, to obtain adeflagration upon ignition.

The pre-blowing pressure is greater than or equal to 10 bars, accordingto the method of execution.

The blowing pressure is, itself, preferably greater than or equal to 30bars.

According to a second aspect, the invention proposes a machine formanufacturing containers from polymer blanks heated in advance,comprising:

-   -   a mold having a cavity defining the final form of the        containers,    -   the means of heating the mold;    -   the means of injection of a pressurized explosive gaseous        mixture into the mold,    -   the means of ignition of the gaseous mixture.

According to one embodiment, the machine comprises a nozzle suitable forcommunicating with the inside of the container and for introducing gasinto it, the means of ignition comprising a spark plug leading into anozzle or, when the machine includes a stretch rod, within it.

Other objects and advantages of the invention would appear to be thelight for the description made here following in reference to thedrawings attached in them.

FIG. 1 is a differential calorimetric analysis (DSC) thermogramillustrating the thermal capacity variations of an initially amorphousPET;

FIG. 2 is a schematic view of a cross-section elevation view showing amachine for manufacturing containers by stretch-blowing;

FIGS. 3A to 3F are schematic cross-section elevation views showingdifferent successive stages of a manufacturing process bystretch-blowing of containers, according to a first example ofexecution;

FIG. 4A is a graph illustrating the development over time of thepressure prevailing in the container at the time of its stretch-blowingin accordance with the process illustrated in FIGS. 3A to 3F;

FIG. 4B is a chronogram illustrating the opening and closing of theelectromagnetic valves as well as the lighting operation within themachine illustrated on FIG. 2, for the implementation of the processillustrated in FIGS. 3A to 3F and in FIG. 4A;

FIGS. 5A to 5H are schematic cross-section elevation views showingdifferent successive stages of a manufacturing process bystretch-blowing of containers, according to a second example ofexecution;

FIG. 6A is a graph illustrating the development over time of thepressure prevailing in the container at the time of its stretch-blowingin accordance with the process illustrated in FIGS. 5A to 5H;

FIG. 6B is a chronogram illustrating the opening and closing of theelectromagnetic valves as well as the lighting operation within themachine illustrated in FIG. 2, for the implementation of the processillustrated in FIGS. 5A to 5H and in FIG. 6A;

FIGS. 7A to 7L are schematic cross-section elevation views showingdifferent successive stages of a manufacturing process bystretch-blowing of containers, according to a third example ofexecution;

FIG. 8A is a graph illustrating the development over time of thepressure prevailing in the container at the time of its stretch-blowingin accordance with the process illustrated in FIGS. 7A to 7L;

FIG. 8B is a chronogram illustrating the opening and closing of theelectromagnetic valves as well as the lighting operation within themachine illustrated on FIG. 2, for the implementation of the processillustrated in FIGS. 7A to 7L and in FIG. 8A;

FIGS. 9A to 9K are schematic cross-section elevation views showingdifferent successive stages of a manufacturing process bystretch-blowing of containers, according to a fourth example ofexecution;

FIG. 10A is a graph illustrating the development over time of thepressure prevailing in the container at the time of its stretch-blowingin accordance with the process illustrated in FIGS. 9A to 9K;

FIG. 10B is a chronogram illustrating the opening and closing of theelectromagnetic valves as well as the lighting operation within themachine illustrated in FIG. 2, for the implementation of the processillustrated in FIGS. 9A to 9K and on FIG. 10A;

FIGS. 11A to 11F are schematic cross-section elevation views showingdifferent successive stages of a manufacturing process bystretch-blowing of containers, according to a fifth example ofexecution;

FIG. 12A is a graph illustrating the development over time of thepressure prevailing in the container at the time of its stretch-blowingin accordance with the process illustrated in FIGS. 11A to 11F;

FIG. 12B is a chronogram illustrating the opening and closing of theelectromagnetic valves of a machine implementing the process illustratedin FIG. 12A;

FIG. 13 is a top view showing a typical example of a container obtainedby a manufacturing process according to any one of the examplesillustrated on the preceding figures;

FIG. 14 is an enlarged scale sectional view of a detail of the body ofthe container of FIG. 13, taken in the inset XIV;

FIG. 15 is a thermogram of differential calorimetric analysis (DSC)illustrating the variations of the mass thermal capacity of a containermanufactured according to a manufacturing process according to one ofthe examples illustrated in the preceding figures, for five successivesections of the wall of the container; and

FIG. 16 is a thermogram similar to that of FIG. 14, for a containermanufactured according to a manufacturing process according to anotherof the examples illustrated in the preceding figures.

FIG. 2 partially illustrates a machine 1 for manufacturing containers 2by stretch-blowing from polymer blanks 3. There following, the blanks 3are the preforms: this term is used for purposes of consistency.

Machine 1 comprises a plurality of molding units 4, mounted on acarousel (not shown), each comprising one mold 5 (as illustrated in FIG.2). This mold 5, made of steel or aluminum alloy, comprises two diehalves 6 and a mold base 7 that together define an internal cavity 8,intended to receive a preform 3 previously heated to a temperaturegreater than the vitreous transition temperature (Tg) of the mattercomprising the preform 3, the shape of which corresponds to the finaldesired shape of the container 2 manufactured from this preform 3.Whatever its shape, container 2 (as illustrated in FIG. 13) generallycomprises a neck 9, a body 10 and a bottom 11.

The molding unit 4 comprises:

-   -   a stretch rod 12 equipped with holes 13 and mounted sliding in        relation to the mold 5 along the main axis 14 (generally        revolving) thereof, between an upper position permitting the        introduction of the preform 3 and a lower position where, at the        end of the stretching of the preform 3, the rod 12 reaches the        bottom of the mold 7, pinning it flat on the bottom of it 11,    -   a casing 15 defining a nozzle 16 into which the rod 12 slides        and which, at the time of the manufacture of the container 2,        engages with a neck 17 of the preform 3 (joined with the neck 9        of the container and not sustaining deformation during the        manufacture),    -   several fluidic circuits run into the nozzle 16 via casing 15,        that is:    -   a medium pressure pre-blowing air circuit 18 (between 5 and 20        bars), this circuit 18 comprising a pre-blowing air source 19        (for example a first compressed air line supplying several        machines) and a conduit 20 (that can be established at least        partially in the casing 15) connecting this source 19 to the        nozzle 16 with the interposition of a first electromagnetic        valve EV1,    -   a reactive gas circuit 22 (gas examples will be given here        following) comprising a reactive gas source 23 and a conduit 24        (which can be formed at least partially in the casing 15)        connecting this source 23 to the nozzle 16 with the        interposition of a second electromagnetic valve EV2,    -   a high pressure blowing air circuit 26 (between 30 and 40 bars),        comprising a blowing air source 27 (for example a second        compressed air line supplying several machines) and a conduit 28        (that can be established at least partially in the casing 15)        connecting this source 27 to the rod 12 with the interposition        of a third electromagnetic valve EV3,    -   a degassing circuit 30 comprising a vent orifice 31 (possibly        doubled from a pump) and a conduit 32 connecting the nozzle to        this orifice with the interposition of a fourth electromagnetic        valve EV4.

The reactive gases can be hydrogen (H₂), methane (CH₄), propane (C₃H₈)or acetylene (C₂H₂).

Hydrogen is preferred due to the non-polluting character of itsoxidation reaction (2H₂+O_(2→)2H₂O), the product of which is pure water.Hydrogen can either be produced on demand, directly upstream of themachine 1 (for example by electrolysis of the water), or stored incontainers from which it is drawn for the needs of the process.

The conduits 20, 24, 28, 32 can be established at least partially in thecasing 15, as is illustrated in FIG. 2. As for the electromagneticvalves EV1, EV2, EV3, EV4, they are connected electrically to a controlunit 34 that controls the opening and closing of them (duly taking intoaccount the response time of the electromagnetic valves). Theseelectromagnetic valves EV1, EV2, EV3, EV4 can be arranged at a distancefrom the casing 15 or, integrated within it for greater compactness. Tomake such a casing 15, a person skilled in the art could refer to patentapplication FR 2 872 082 (Sidel) or to the equivalent internationalpatent application WO 2006/008380.

The state of electromagnetic valves EV1, EV2, EV3, EV4 (closed/open) andof the spark plug 36 (out/lit) is illustrated in the chronograms ofFIGS. 4B, 6B, 8B, 10B, 12B, each consisting of five lines, numbered 1 to5 and respectively representing:

-   -   line 1, the state (0: closed/1: open) of the first        electromagnetic valve (pre-blowing air supply),    -   line 2, the state (0: closed/1: open) of the second        electromagnetic valve (reactive gas supply),    -   line 3, the state (0: out/1: lit) of the spark plug, line 4, the        state (0: closed/1: open) of the third electromagnetic valve        (blowing air supply),    -   line 5: the state (0: closed/1: open) of the fourth        electromagnetic valve (degassing).

As seen in FIG. 2, the molding unit 4 is in addition equipped with adevice for its own ignition 35, at a pre-set given instant and actuatedby the control unit 34, to produce within the nozzle 16 (or of thecontainer 2) a spark for igniting the air and reactive gas mixture inthe container 2.

According to an embodiment, this ignition device 35 comprises a sparkplug 36 having a center electrode 37 and an earth electrode 38 bothleading into the nozzle 16 (that communicates with the inside of thecontainer 2)—or, as a variant, in the rod 12—and between which, uponactuation by control unit 34, an electrical arc can be produced, causingthe ignition of the mixture.

As illustrated in FIG. 2, molding unit 4 is in addition equipped with acircuit 39 for heating mold 5, comprising a pressurized coolant source40 (by oil or water, for example) and conduits 41 arranged in thethickness of the mold 5 (half dies 6 and bottom 7 included), in whichthe coolant fluid coming from source 40 circulates to keep mold 5 at atemperature above the ambient temperature (20° C.). In practice, thetemperature of the mold is regulated at an average value between 20° C.and 160° C. (measured on the inside wall of mold 5), depending on theapplications—examples of temperatures are provided here following.

Here following we describe five specific examples of manufacturingprocesses for containers 2 of the HR (heat resistant) type having forexample a shape such as that illustrated in FIG. 13, by means of machine1, which has just been described.

For each example described, DSC is used to measure the crystallinity ofa container 2 obtained by the corresponding process. More precisely, thecrystallinity of body 10 of container 2 is measured, at least on theside of an inside wall 42 and of an outside wall 43. For this purpose asample is taken in the body 10 and is cut (by microtomic cutting, forexample) into serial segments at its thickness, the respectivecrystallinity of which is then measured. According to one embodiment,the sample is cut into five approximately equal segments. For example,for a container 2, the thickness of which is approximately 360 μm, eachcut segment exhibits a thickness of 50 μm (the cutting blade forming,with each pass between two successive segments, shavings of a thicknessof approximately 25 μm). A, B, C, D and E show the five successivesegments of material, from the insider of container 2.

We would like to point out that the DSC analysis makes it possible toquantify the thermal phenomena (endo or exothermic) accompanying thetransformations (crystallization, fusion) of a material.

The procedure used here is as follows.

A differential microcalorimeter with power compensation is used. Thismicrocalorimeter includes two ovens under a neutral atmosphere(generally in nitrogen). A reference (generally an empty cup) is placedinto the first. The sample on which the DSC measurements are to be madeis placed into the second.

Each oven is equipped with two platinum resistances, one of which servesfor the heating and the other for measuring the temperature.

The exchanged heat fluxes are measured, on the one hand between thereference and the outside medium, and on the other hand between thesample and the outside medium as the temperatures is increased at aconstant heating velocity from the ambient temperature (about 20° C.) upto a temperature greater than the known fusion temperature of thestudied material (in this case, for the PET it is heated up to about300° C., it being assumed that the material fuses at approximately 250°C.).

The mass thermal capacity is deduced from the studied sample bycomparison with the reference, by the following relationship:

$C_{p} = {\frac{1}{m}\frac{Q}{T}}$

Where:

m is the mass of the sample in grams,

T the temperature,

Q the amount of heat in J.g⁻¹.K⁻¹ necessary to cause an increase of thetemperature in the sample of the value dT.

If the heating velocity q is introduced, kept constant at the time ofthe measurement (and in the selected case equal to 10 K⁻¹), defined bythe relationship

${q = \frac{Q}{t}},$

then the mass thermal capacity can be written as follows:

$C_{p} = {\frac{1}{m \cdot q}\frac{Q}{t}}$

The variations of the mass thermal capacity of the sample in relation tothe temperature are traced from the measurements of the heat fluxperformed in the microcalorimeter. The curve of these variations iscalled a thermogram. Such a thermogram is shown in FIG. 1 for a sampleof the PET of the manufacturer EASTMANN mentioned in the introduction.

Such a thermogram makes it possible to differentiate the exothermicphenomena (oriented downwards) from the endothermic phenomena (orientedupwards).

For any initially amorphous PET sample, such as the above samplementioned, one finds two peaks: a first exothermic peak (in this casearound 135° C.), corresponding to the crystallization of the material,and an endothermic peak (in this case around 250° C.) corresponding tothe fusion of the material.

The rate of crystallinity of the initial material can be calculated fromthe thermogram, from the difference ΔH of the enthalpies exchangedduring the fusion phenomena on the one hand, and the crystallization onthe other.

The fusion enthalpy ΔH_(f) is defined by the area under the fusion peak:

Δ H_(f) = ∫_(pic  fusion)C_(p)(T)T

The crystallization enthalpy ΔH_(c), is itself defined by the area underthe crystallization peak:

Δ H_(c) = −∫_(pic  cristallisation)C_(p)(T)T

The differences of the fusion and the crystallization enthalpy arededuced from: ΔH=ΔH_(f)−ΔH_(c), then the crystallinity rate from thefollowing relationship:

$X_{C} = \frac{\Delta \; H}{H_{ref}}$

Where H_(ref) is the enthalpy of fusion of a presumed completelycrystalline sample. Here a value of 140 J.g⁻¹ is selected, whichcorresponds to the most value most commonly used in plastic materialslaboratories.

EXAMPLE 1 FIGS. 3A to 3F, 4A and 4B)

In this example, mold 5 is heated such that such that it exhibits on theside of its inside wall a temperature of approximately 160° C. Thematerial of the preform 3 is a PET. The reactive gas is hydrogen (H₂).The air/hydrogen gaseous mixture is made while maintaining a hydrogenproportion in volume between 4% and 18%, preferably 6%.

Following introduction of the hot preform 3 into the mold 5, the processcomprises a first operation, known as pre-blowing, consisting ofstretching preform 3 by sliding rod 12, and simultaneously opening theelectromagnetic valves EV1 and EV2 to introduce into preform 3 an airand reactive gas mixture at a pressure between approximately 5 and 20bars (FIGS. 3A to 3C, lines 1 and 2 in FIG. 4B). This first operation ofa predetermined durational α1 (less than 250 ms) ends by the closing ofthe electromagnetic valves EV1 and EV2 after rod 12 has ended its travelhaving reached the bottom of mold 7, and the threshold for the plasticoutflow of the material has been exceeded (which is indicated on thepressure curve by a first pressure peak, at approximately 8 to 10 bars).

A second operation, known as ignition, consists of igniting the gaseousmixture by ignition of a spark plug 36 (FIG. 3D, line 3 in FIG. 4B).Taking into account the proportion of hydrogen in the gaseous mix, anexplosion occurs in container 2 that is being formed, which isaccompanied by a sudden increase of the temperature (which reacheshundreds of degrees Celsius) and of the pressure (which exceeds 40bars—on the curve of FIG. 4A; the corresponding peak pressure is clippeddue to reasons of scale). The duration of the α2 of the ignition of themixture is extremely brief (less than 25 ms), but the increased pressurethat accompanies it is sufficient to pin the substance flat against thewall of the mold, thus forming container 2.

A third operation, known as stabilization, consists of maintaining incontainer 2 a residual gas (essentially a mixture of water vapor comingfrom the combustion of hydrogen, with possible traces of NO_(x)), for apredetermined duration α3 (between 1000 and 1500 ms) while keeping allthe electromagnetic valves EV1, EV2, EV3, EV4 closed, so as to permitthe reduction of the temperature and the pressure in container 2 (FIG.3E).

A fourth operation, known as degassing, consists of degassing container2, by closing third electromagnetic valve EV3 (FIG. 4B, line 4) andleaving fourth electromagnetic valve EV4 for a preset duration α4(between 100 and 500 ms), to allow the air to escape (FIG. 3F) until thepressure prevailing inside container 2 has about attained theatmospheric pressure (FIG. 4B, line 5). At the end of this operation,the fourth electromagnetic valve EV4 is closed, the mold 5, opened, andcontainer 2, evacuated, to enable repetition of the cycle with a newpreform 3.

DSC was used, in the above described conditions to measure thecrystallinity of a container 2 obtained by this process, on the side ofinside wall 42 of body 10 (segment A) and of the side of its outsidewall 43 (segment E). The results of the measurements are presented inthe following table:

Segment Crystallinity (%) A (inside wall) 22 E (outside wall) 32

It is apparent that container 2 exhibits a negative gradient ofcrystallinity in the area of its inside wall 42. The crystallinitymeasured from the side of inside wall 42 is in this case much less(about 30%) than the crystallinity measured from the side of outsidewall 43.

It may also be noted that the mechanical resistance to deformation of acontainer 2, at the time of a hot filling (with a liquid, thetemperature of which is between 85° C. and 95° C.), is greater than thatof a container obtained by a process without ignition (See, thecomparative example). In fact, for a liquid temperature between 85° C.and 95° C., the retraction rate of the container is less than or equalto 1%.

This phenomenon can be explained as follows. The extreme temperatureconditions (that reach several hundred degrees Celsius) and the pressureprevailing within container 2 over the course of the formation at thetime of the deflagration induced by the ignition of the explosivegaseous mixture cause the fusion of the material at least of the side ofthe inside wall 42 of container 2.

Following the deflagration, formed container 2 undergoes a heat setting,from the side of its outside wall 43 in contact with the heated wall ofmold 5. It thus benefits, over a certain thickness of its outside wall43, from a contribution of crystallinity by thermal means, while itsinside wall 42, which becomes completely (or almost completely)amorphous following its fusion, retains a high proportion of amorphousmatter, with a comparatively lower rate of crystallinity.

Despite the low rate of crystallinity of the side of inside wall 42, thehigh rate of crystallinity of the side of outside wall 43 givescontainer 2 a rigidity equivalent to that of a container with constantcrystallinity (such as a simply thermoset container, obtained by theprocess described in the comparative example, that is to say withoutigniting an explosive gaseous mixture), the portion of highcrystallinity matter (comprising outside wall 43) acting in the mannerof a brace with respect to the portion of low crystallinity matter(comprising inside wall 42).

In this example, the thickness of inside wall 42 corresponds to thethickness of segment A, as cut out for the needs of the DSC analysis(See above). Measurements have shown that the gradient of crystallinitydoes not extend beyond segment C. Consequently, inside wall 42 affectedby the negative gradient of crystallinity exhibits a thickness less than100 μm, and more likely less than approximately 50 μm.

During the hot filling, the residual stresses stored by container 2 atthe time of its formation are largely released into the amorphous matterpresent in a large proportion of the side of inside wall 42, which thusacts as a buffer versus the portion of high crystallinity matter,preventing the propagation of deformations in the container.

EXAMPLE 2 FIGS. 5A to 5H, 6A and 6B)

In this example, mold 5 is heated such that such that it exhibits on theside of its inside wall a temperature of approximately 160° C. Thematerial of the preform 3 is a PET. The reactive gas is hydrogen (H₂).The air/hydrogen gaseous mixture is made while maintaining a hydrogenproportion in volume between 4% and 18%, preferably 6%.

Following introduction of the hot preform 3 into mold 5, a firstpre-blowing operation, consisting of stretching preform 3 by sliding rod12, and to simultaneously pre-blow it by opening the electromagneticvalves EV1 and EV2 to introduce into preform 3 an air and reactive gasmixture at a pressure between approximately 5 and 20 bars (FIGS. 5A to5C, lines 1 and 2 on FIG. 6B). This first operation of a predeterminedduration β1 (less than 250 ms) ends by the closing of theelectromagnetic valves EV1 and EV2 after rod 12 has ended its travelhaving reached the bottom of mold 7, and the threshold for the plasticoutflow of the material has been exceeded (which is indicated on thepressure curve by a first pressure peak, at approximately 8 to 10 bars).

A second operation, known as ignition, consists of igniting the gaseousmixture by ignition of spark plug 36 (FIG. 5D, line 3 in FIG. 6B).Taking into account the proportion of hydrogen in the gaseous mix, adeflagration occurs in container 2 that is being formed, which isaccompanied by an abrupt increase of the temperature (which reacheshundreds of degrees Celsius) and of the pressure (which exceeds 40bars—on the curve of FIG. 6A the corresponding peak pressure is clippeddue to reasons of scale). The duration β2 of the ignition of the mixtureis extremely brief (less than 25 ms), but the increased pressure thataccompanies it is sufficient to pin the matter flat against the wall ofmold 5, thus forming container 2.

A third operation, known as stabilization, consists of maintaining incontainer 2 the residual gas (essentially a mixture of water vaporcoming from the combustion of hydrogen, with possible traces of NO_(x)),for a predetermined duration β3 (between 200 and 300 ms) while keepingall the electromagnetic valves EV1, EV2, EV3, EV4 closed, so as topermit the reduction of the temperature and the pressure in container 2(FIG. 5E).

A fourth operation, known as blowing, consists of opening thirdelectromagnetic valve EV3 to introduce into container 2, via holes 13arranged in rod 12, high pressure air (between approximately 30 and 40bars) at ambient temperature and thus keep pinned flat against the wallof mold 5 container 2 formed at the time of the ignition operation (FIG.5F, line 4 on FIG. 6B). During this blowing operation, of apredetermined duration β4 (preferably less than 1000 ms), the fourthelectromagnetic valveEV4 is kept closed.

A fifth operation, known as sweeping, consists of making an air sweep ofcontainer 2, while keeping third electromagnetic valve EV3 open tocontinue introducing high pressure air (FIG. 6B, line 4) at ambienttemperature into container 2, while simultaneously opening fourthelectromagnetic valve EV4 to permit pressurized air to be evacuated(FIG. 6B, line 5). An air circulation is thus generated in container 2while maintaining it under pressure (between 10 and 15 bars), whichcontinues to pin it flat against the wall of mold 5 while cooling it (atleast on the same side of its inside wall 42), so that upon emergingfrom mold 5 it retains the shape that the latter gives it (FIG. 5G).This sweep operation is performed for a predetermined duration β5,between 200 et 2000 ms.

A sixth operation, known as degassing, consists of degassing container2, by closing third electromagnetic valve EV3 (FIG. 6B, line 4) andleaving fourth electromagnetic valve EV4 open for a preset duration β6(between 100 and 500 ms), to allow the air to escape (FIG. 5H) until thepressure prevailing inside container 2 has about attained theatmospheric pressure (FIG. 6B, line 5). At the end of this operation,the fourth electromagnetic valve EV4 is closed, mold 5 opened andcontainer 2 evacuated to enable repetition of the cycle with a newpreform.

DSC was used, in the above described conditions to measure thecrystallinity of a container 2 obtained by this process, on the side ofinside wall 42 of body 10 (segment A) and of the side of its outsidewall 43 (segment E). The results of the measurements are presented inthe following table:

Segment Crystallinity (%) A (inside wall) 22 E (outside wall) 32

It is apparent that container 2 exhibits a negative gradient ofcrystallinity in the area of its inside wall 42. The crystallinitymeasured from the side of inside wall 42 is in this case much less(about 30%) than the crystallinity measured from the side of outsidewall 43.

It may also be noted that the mechanical resistance to deformation of acontainer 2, at the time of a hot filling (with a liquid, thetemperature of which is between 85° C. and 95° C.), is greater than thatof a container obtained by a process without ignition (See, thecomparative example). In fact, for a liquid temperature between 85° C.and 95° C., the retraction rate of the container is less than or equalto 1%.

This phenomenon can be explained as follows. The extreme temperatureconditions (that reach several hundred degrees Celsius) and the pressureprevailing within container 2 over the course of the formation at thetime of the deflagration induced by the ignition of the explosivegaseous mixture cause the fusion of the material at least of the side ofthe inside wall 42 of container 2.

Following the deflagration, formed container 2 undergoes a heat setting,from the side of its outside wall 43 in contact with the heated wall ofmold 5. It thus benefits, over a certain thickness of its outside wall43, from a contribution of crystallinity by thermal means, while itsinside wall 42, which becomes completely (or almost completely)amorphous following its fusion, retains a high proportion of amorphousmatter, with a comparatively lower rate of crystallinity.

Despite the low rate of crystallinity of the side of inside wall 42, thehigh rate of crystallinity of the side of outside wall 43 givescontainer 2 a rigidity equivalent to that of a container with constantcrystallinity (such as a simply thermoset container, obtained by theprocess described in the comparative example, that is to say withoutigniting an explosive gaseous mixture), the portion of highcrystallinity matter (comprising outside wall 43) acting in the mannerof a brace with respect to the portion of low crystallinity matter(comprising inside wall 42).

In this example, the thickness of inside wall 42 corresponds to thethickness of segment A, as cut out for the needs of the DSC analysis(See above). Measurements have shown that the gradient of crystallinitydoes not extend beyond segment C. Consequently, inside wall 42 affectedby the negative gradient of crystallinity exhibits a thickness less than100 μm, and more likely less than approximately 50 μm.

During the hot filling, the residual stresses stored by container 2 atthe time of its formation are largely released into the amorphous matterpresent in a large proportion of the side of inside wall 42, which thusacts as a buffer versus the portion of high crystallinity matter,preventing the propagation of deformations in the container.

EXAMPLE 3 FIGS. 7A to 7L, 8A and 8B)

In this example mold 5 is heated such that it exhibits on the side ofits inside wall a temperature of approximately 130° C. The material ofthe preform 3 is a PET. The reactive gas is hydrogen (H₂). Theair/hydrogen gaseous mixture is made while maintaining a hydrogenproportion in volume between 4% and 18%, preferably 6%.

Following introduction of the hot preform 3 into mold 5, a firstoperation, known as pre-blowing, consists of stretching preform 3 bysliding rod 12, and simultaneously pre-blowing it by openingelectromagnetic valves EV1 and EV2 to introduce into preform 3 an airand reactive gas mixture at a pressure between approximately 5 and 20bars (FIGS. 7A to 7C, lines 1 and 2 on FIG. 8B). This first operation ofa predetermined duration γ1 (less than 250 ms) ends by the closing ofthe electromagnetic valves EV1 and EV2 after rod 12 has ended its travelhaving reached the bottom of mold 7, and the threshold for the plasticoutflow of the material has been exceeded (which is indicated on thepressure curve by a first pressure peak, at approximately 8 to 10 bars).

A second operation, known as primary ignition, consists of igniting thegaseous mixture by ignition of spark plug 36 (FIG. 7D, line 3 in FIG.8B). Taking into account the proportion of hydrogen in the gaseous mix,a deflagration occurs in container 2 that is being formed, which isaccompanied by an abrupt increase of the temperature (which reacheshundreds of degrees Celsius) and of the pressure (which exceeds 40bars—on the curve of FIG. 7A; the corresponding peak pressure is clippeddue to reasons of scale). The duration of γ2 of the ignition of themixture is extremely brief (less than 25 ms), but the increased pressurethat accompanies it is sufficient to pin the substance flat against thewall of mold 5, thus forming container 2.

A third operation, known as primary stabilization, consists ofmaintaining in container 2 a residual gas (essentially a mixture ofwater vapor coming from the combustion of hydrogen, with possible tracesof NO_(x)), for a predetermined duration γ3 (between 200 and 300 ms)while keeping all the electromagnetic valves EV1, EV2, EV3, EV4 closed,so as to permit the reduction of the temperature and the pressure incontainer 2 (FIG. 8E).

A fourth operation, known as degassing, consists of degassing container2, by closing the fourth electromagnetic valve EV4 for a preset duration4 (between 100 and 200 ms), to allow the air to escape (FIG. 7F) untilthe pressure prevailing inside container 2 has attained about theatmospheric pressure (FIG. 8B, line 5).

A fifth operation, known as secondary pre-blowing, consists ofre-opening electromagnetic valves EV1 and EV2 to introduce into thecontainer an air and reactive gas mixture at a pressure betweenapproximately 5 and 20 bars (FIG. 7G, lines 1 and 2 in FIG. 8B). Thisfifth operation, of a predetermined duration γ5 (less than 250 ms) endsby the closing of the electromagnetic valves EV1 et EV2 after thepressure in container 2 has reached a value between 5 and 20 bars.

A sixth operation, known as ignition, consists of igniting the gaseousmixture by ignition of spark plug 36 (FIG. 7H, line 3 in FIG. 8B).Taking into account the proportion of hydrogen in the mixture, adeflagration is produced in container 2, which is accompanied by anabrupt increase in the temperature (which again reaches several hundreddegrees Celsius) and the pressure (which again exceeds 40 bars, thecorresponding peak pressure likewise being clipped in FIG. 8A). As inthe primary ignition operation, the duration γ6 of the ignition of themixture is extremely brief (less than 25 ms).

A seventh operation, known as secondary stabilization, consists ofmaintaining in container 2 a residual gas (essentially a mixture ofwater vapor coming from the combustion of hydrogen, with possible tracesof NO_(x)), for a predetermined duration γ7 (between 200 and 300 ms),while keeping all the electromagnetic valves EV1, EV2, EV3, EV4 closed,so as to permit the reduction of the temperature and the pressure incontainer 2 (FIG. 7I).

An eighth operation, known as blowing, consists of opening the thirdelectromagnetic valve EV3 to introduce into container 2, via holes 13arranged in rod 12, high pressure air (between approximately 30 and 40bars) at ambient temperature and thus keep pinned flat against the wallof mold 5 container 2 formed at the time of the ignition operation (FIG.7J, line 4 on FIG. 8B). During this blowing operation, of apredetermined duration γ8 (preferably less than 300 ms), the fourthelectromagnetic valve EV4 is kept closed.

A ninth operation, known as sweeping, consists of making an air sweep ofthe container, while keeping the third electromagnetic valve EV3 open tocontinue introducing high pressure air (FIG. 8B, line 4) at ambienttemperature into container 2, while simultaneously opening the fourthelectromagnetic valve EV4 to permit pressurized air to be evacuated(FIG. 8B, line 5). An air circulation is thus generated in container 2while maintaining it under pressure (between 10 and 15 bars), whichcontinues to pin it flat against the wall of mold 5 while cooling it (atleast on the side of its inside wall 42), so that upon emerging frommold 5 it retains the shape that the latter gives it (FIG. 7K). Thissweep operation is performed for a predetermined duration γ9, between200 et 2000 ms.

A tenth operation, known as secondary degassing, consists of degassingcontainer 2, by closing the third electromagnetic valve EV3 (FIG. 8B,line 4) and leaving the fourth electromagnetic valve EV4 for a presetduration γ10 (between 100 and 500 ms), to allow the air to escape (FIG.7L) until the pressure prevailing inside container 2 has about attainedatmospheric pressure (FIG. 8B, line 5). At the end of this operation,the fourth electromagnetic valve EV4 is closed, mold 5 opened andcontainer 2 evacuated to enable repetition of the cycle with a newpreform.

DSC was used, in the above described conditions, to measure thecrystallinity in the thickness of a container 2 obtained by thisprocess. The results of the measurements are presented in the followingtable:

Segment Crystallinity (%) A (inside wall) 15 B 30 C 26 D 27 E (outsidewall) 28

It is apparent that container 2 exhibits a negative gradient ofcrystallinity in the area of its inside wall 42. The crystallinitymeasured from the side of inside wall 42 is in this case much less(about 50%) than the crystallinity measured from the side of outsidewall 43.

It may also be noted that the mechanical resistance to deformation ofsuch a container 2, at the time of a hot filling (with a liquid, thetemperature of which is between 85° C. and 95° C.), is greater than thatof a container obtained by a process without ignition (See thecomparative example). In fact, for a liquid temperature between 85° C.and 95° C., the retraction rate of the container is less than or equalto 1%.

This phenomenon can be explained as follows. The extreme temperatureconditions (that reach several hundred degrees Celsius) and the pressureprevailing within container 2 over the course of the formation at thetime of the deflagration induced by the ignition of the explosivegaseous mixture cause the fusion of the material at least on the side ofthe inside wall 42 of container 2.

Following the deflagrations, formed container 2 undergoes a heatsetting, of the side of its outside wall 43 in contact with the heatedwall of mold 5. It thus benefits, over a certain thickness of itsoutside wall 43, from a contribution of crystallinity by thermal means,while its inside wall 42, which becomes completely (or almostcompletely) amorphous following its fusion, retains a high proportion ofamorphous matter, with a comparatively lower rate of crystallinity.

Despite the low rate of crystallinity of the side of inside wall 42, thehigh rate of crystallinity of the side of outside wall 43 givescontainer 2 a rigidity equivalent to that of a container with constantcrystallinity (such as a simply thermoset container, obtained by theprocess described in the comparative example, that is to say withoutigniting an explosive gaseous mixture), the portion of highcrystallinity matter (comprising outside wall 43) acting in the mannerof a brace with respect to the portion of low crystallinity matter(comprising inside wall 42).

In this example, the thickness of inside wall 42 corresponds to thethickness of segment A, as cut out for the needs of the DSC analysis(See above). Measurements have shown that the gradient of crystallinitydoes not extend beyond segment C. Consequently, inside wall 42 affectedby the negative gradient of crystallinity exhibits a thickness less than100 μm, and more likely less than approximately 50 μm.

During the hot filling, the residual stresses stored by container 2 atthe time of its formation are largely released into the amorphous matterpresent in a large proportion of the side of inside wall 42, which thusacts as a buffer versus the portion of high crystallinity matter,preventing the propagation of deformations in the container.

In order to verify the amorphous character of layer A, a thermalanalysis was performed on this container 2 by means of DSC, by taking asample similar to that used for measuring the crystallinity, and bycutting it in the same manner to obtain five similar segments A, B, C, Dand E. The DSC curves of the five segments are consolidated on thethermogram of FIG. 15. It is apparent that the curves of segments B, C,D and E all exhibit a single peak endothermic fusion around 250° C.,while the curve of segment A exhibits, in addition to this sameendothermic peak around 125° C., an exothermic peak around 250° C.

This endothermic peak, found between the vitreous transition temperature(occurring around 80° C.) and the fusion peak, is a crystallizationpeak, attesting to the amorphous character of the matter of segment A,of the side of inside wall 42 of container 2. Conversely, the absence ofsuch a crystallization peak on the curves of the other segments B to Eattests to the semi-crystalline character of the matter in particular ofthe side of the outside wall 43. In other words, container 2 can beconsidered, at the end of its manufacture, to be amorphous on the sideof its inside wall 42.

By comparison, the same analysis was performed on a sample taken from acontainer obtained by the process described in the comparative example(that is to say without igniting an explosive gaseous mixture). It isapparent (FIG. 16) that the DSC curves of the five segments A to E ofthe sample exhibit only one peak, endothermic, corresponding to thefusion of the matter at about 250° C., indicating that the container issemi-crystalline throughout its thickness, including on the side of itsinternal wall.

EXAMPLE 4 FIGS. 9A to 9K, 10A and 10B)

In this example mold 5 is heated such that it exhibits on the side ofits inside wall a temperature of approximately 130° C. The material ofthe preform 3 is a PET. The reactive gas is hydrogen (H₂). Theair/hydrogen gaseous mixture is made while maintaining a hydrogenproportion in volume between 4% and 18%, preferably 6%.

Following introduction of the hot preform 3 into mold 5, a firstoperation, known as primary pre-blowing, consists of stretching preform3 by sliding rod 12, and simultaneously pre-blowing it by opening firstelectromagnetic valve EV1 to introduce into preform 3 an air andreactive gas mixture at a pressure between approximately 5 and 20 bars(FIGS. 9A to 9C, line 1 on FIG. 10B). This first operation of apredetermined duration δ1 (less than 250 ms) ends by the closing ofelectromagnetic valve EV1 and after rod 12 has ended its travel, havingreached mold bottom 7, and the threshold for the plastic outflow of thematerial has been exceeded (which is indicated on the pressure curve bya first pressure peak, at approximately 8 to 10 bars).

A second operation, known as blowing, consists of blowing container 2 byopening the third electromagnetic valve EV3 to introduce into container2 being formed, via holes 13 arranged in rod 12, high pressure air(between approximately 30 and 40 bars) at ambient temperature, so as tokeep container 2 pinned flat against the wall of mold 5 (FIG. 9D, line 4on FIG. 10B). During this blowing operation, of a predetermined durationδ2 (preferably less than 300 ms), fourth electromagnetic valve EV4 iskept closed.

A third operation, known as degassing, consists of degassing container2, by opening the fourth electromagnetic valve EV4 for a preset durationδ3 (between 100 and 200 ms), to allow the air to escape (FIG. 9E) untilthe pressure prevailing inside container 2 has attained about theatmospheric pressure (FIG. 10B, line 5).

A fourth operation, known as secondary pre-blowing, consists ofre-opening electromagnetic valves EV1 and EV2 to introduce intocontainer 2 an air and reactive gas mixture at a pressure betweenapproximately 5 and 20 bars (FIG. 9F, lines 1 and 2 on FIG. 10B). Thisfourth operation, of a predetermined duration δ4 (less than 250 ms) endsby the closing of electromagnetic valves EV1 and EV2 after the pressurein container 2 has reached a value between 5 and 20 bars.

A fifth operation, known as ignition, consists of igniting the gaseousmixture by ignition of spark plug 36 (FIG. 9G, line 3 in FIG. 10B).Taking into account the proportion of hydrogen in the gaseous mixture, adeflagration occurs in container 2, accompanied by an abrupt increase ofthe temperature (which reaches several hundred degrees Celsius) and ofthe pressure (which exceeds 40 bars—on the curve of FIG. 10A; thecorresponding peak pressure is clipped due to reasons of scale). Theduration δ5 of ignition of the mixture is extremely brief (less than 25ms).

A sixth operation, known as stabilization, consists of maintaining incontainer 2 a residual gas (essentially a mixture of air and water vaporcoming from the combustion of hydrogen, with possible traces of NO_(x)),for a predetermined duration δ6 (between 200 and 300 ms), while keepingall the electromagnetic valves EV1, EV2, EV3, EV4 closed, so as topermit the reduction of the temperature and the pressure in container 2(FIG. 9H).

An seventh operation, known as secondary blowing, consists of openingthe third electromagnetic valve EV3 to introduce into container 2, viaholes 13 arranged in rod 12, high pressure air (between approximately 30and 40 bars) at ambient temperature and thus keep pinned flat againstthe wall of mold 5 container 2 formed at the time of the ignitionoperation (FIG. 9I, line 4 on FIG. 10B). During this blowing operation,of a predetermined duration δ7 (preferably less than 300 ms), fourthelectromagnetic valve EV4 is kept closed.

An eighth operation, known as sweeping, consists of making an air sweepof container 2, while keeping third electromagnetic valve EV3 open tocontinue introducing high pressure air (FIG. 10B, line 4) at ambienttemperature into container 2, while simultaneously opening the fourthelectromagnetic valve EV4 to permit pressurized air to be evacuated(FIG. 10B, line 5). An air circulation is thus generated in container 2,while maintaining it under pressure (between 10 and 15 bars), whichcontinues to pin it flat against the wall of mold 5, while cooling it(at least on the same side of its inside wall), so that upon emergingfrom mold 5 it retains the shape that the latter gives it (FIG. 9J).This sweep operation is performed for a predetermined duration δ8,between 200 et 2000 ms.

A ninth operation, known as secondary degassing, consists of degassingcontainer 2, by closing third electromagnetic valve EV3 (FIG. 10B, line4) and leaving fourth electromagnetic valve EV4 open for a presetduration δ9 (between 100 and 500 ms), to allow the air to escape (FIG.9K) until the pressure prevailing inside container 2 has about attainedatmospheric pressure (FIG. 10B, line 5). At the end of this operation,fourth electromagnetic valve EV4 is closed, mold 5 opened and container2 evacuated to enable repetition of the cycle with a new preform.

DSC was used, in the above described conditions to measure thecrystallinity of a container 2 obtained by this process, on the side ofinside wall 42 of body 10 (segment A) and of the side of its outsidewall 43 (segment E). The results of the measurements are presented inthe following table:

Segment Crystallinity A (inside wall) 25 E (outside wall) 31

It is apparent that container 2 exhibits a negative gradient ofcrystallinity in the area of its inside wall 42. The crystallinitymeasured on the side of inside wall 42 is in this case much less (about20%) than the crystallinity measured on the side of outside wall 43.

It may also be noted that the mechanical resistance to deformation ofsuch a container 2, at the time of a hot filling (with a liquid, thetemperature of which is between 85° C. and 95° C.), is greater than thatof a container obtained by a process without ignition (See thecomparative example). In fact, for a liquid temperature between 85° C.and 95° C., the retraction rate of the container is less than or equalto 1%.

This phenomenon can be explained as follows. The extreme temperatureconditions (that reach several hundred degrees Celsius) and the pressureprevailing within container 2 over the course of the formation at thetime of the deflagration induced by the ignition of the explosivegaseous mixture cause the fusion of the material at least of the side ofthe inside wall 42 of container 2.

Following the deflagration, formed container 2 undergoes a heat setting,of the side of its outside wall 43 in contact with the heated wall ofmold 5. It thus benefits, over a certain thickness of its outside wall43, from a contribution of crystallinity by thermal means, while itsinside wall 42, which becomes completely (or almost completely)amorphous following its fusion, retains a high proportion of amorphousmatter, with a comparatively lower rate of crystallinity.

Despite the low rate of crystallinity of the side of inside wall 42, thehigh rate of crystallinity of the side of outside wall 43 givescontainer 2 a rigidity equivalent to that of a container with constantcrystallinity (such as a simply thermoset container, obtained by theprocess described in the comparative example, that is to say withoutigniting an explosive gaseous mixture), the portion of highcrystallinity matter (comprising outside wall 43) acting in the mannerof a brace with respect to the portion of low crystallinity matter(comprising inside wall 42).

In this example, the thickness of inside wall 42 corresponds to thethickness of segment A, as cut out for the needs of the DSC analysis(See above). Measurements have shown that the gradient of crystallinitydoes not extend beyond segment C. Consequently, inside wall 42 affectedby the negative gradient of crystallinity exhibits a thickness less than100 μm, and more likely less than approximately 50 μm.

During the hot filling, the residual stresses stored by container 2 atthe time of its formation are largely released into the amorphous matterpresent in a large proportion of the side of inside wall 42, which thusacts as a buffer versus the portion of high crystallinity matter,preventing the propagation of deformations in the container.

COMPARATIVE EXAMPLE FIGS. 11A to 11F, 12A, 12B, 16)

In this example, mold 5 is heated such that such that it exhibits on theside of its inside wall a temperature of approximately 160° C. Thematerial of the preform is a PET.

Following introduction of the hot preform 3 into mold 5, a firstoperation, known as pre-blowing, consists of stretching preform 3 bysliding rod 12, and simultaneously pre-blowing it by opening the firstelectromagnetic EV1 to introduce into preform 3 an air and reactive gasmixture at a pressure between approximately 5 and 20 bars (FIGS. 11A to11C, line 1 on FIG. 12B). This first operation of a predeterminedduration ε1 (less than 250 ms) ends by the closing of electromagneticvalve EV1 after rod 12 has ended its travel having reached the bottom ofmold 7, and the threshold for the plastic outflow of the material hasbeen exceeded (which is indicated on the pressure curve by a firstpressure peak, at approximately 8 to 10 bars).

A second operation, known as blowing, consists of blowing preform 3 byopening the third electromagnetic valve EV3 to introduce into preform 3,via holes 13 arranged in rod 12, high pressure air (betweenapproximately 30 and 40 bars) at ambient temperature, so as to pincontainer 2 flat against the wall of mold 5 (FIG. 11D, line 4 on FIG.12B). During this blowing operation, of a predetermined duration ε2(between 500 and 1200 ms), fourth electromagnetic valve EV4 is keptclosed.

A third operation, known as sweeping, consists of making an air sweep ofcontainer 2, while keeping third electromagnetic valve EV3 open tocontinue introducing high pressure air at ambient temperature intocontainer 2 via holes 13 arranged in rod 12 (FIG. 12B, line 4), whilesimultaneously opening the fourth electromagnetic valve EV4 to permitpressurized air to be evacuated (FIG. 12B, line 5). An air circulationis thus generated in container 2 while maintaining it under pressure(between 10 and 15 bars), so as to pin it against mold 5 (FIG. 12E).This sweep operation is performed for a predetermined duration ε3,between 500 et 800 ms.

A fourth operation, known as degassing, consists of degassing container2, by closing the third electromagnetic valve EV3 (FIG. 12B, line 4) andleaving the fourth electromagnetic valve EV4 open for a preset durationε5 (between 200 and 500 ms), to allow the air to escape (FIG. 11F) untilthe pressure prevailing inside container 2 has attained about theatmospheric pressure (FIG. 12B, line 5). At the end of this operation,the fourth electromagnetic valve EV4 is closed, mold 5 opened andcontainer 2 evacuated to enable repetition of the cycle with a newpreform.

DSC was used, in the above described conditions to measure thecrystallinity of a container obtained by this process, from the side ofthe inside wall of the body of the container (segment A) and in the areaof its outside wall (segment E). The results of the measurements arepresented in the following table:

Segment Crystallinity A (inside wall) 31 B 24 C 28 D 35 E (outside wall)30

It is apparent that the crystallinity is more or less constant fromsegment A (inside wall 42) up to segment E (outside wall 43),demonstrating the more or less uniform character, in the thickness ofcontainer 2, of the thermosetting realized by this process.

A thermal analysis of this container 2 is performed by DSC, by taking asample similar to that used for measuring the crystallinity, and bycutting it in the same manner to obtain five similar segments A, B, C, Dand E. The DSC curves of the five segments are consolidated on thethermogram of FIG. 16. It is apparent that the curves all exhibit asingle endothermic peak of fusion around 250° C., attesting to thesemi-crystalline character of the matter throughout the thickness ofcontainer 2.

It may also be noted that the mechanical resistance to deformation ofsuch a container 2, at the time of a hot filling (with a liquid, thetemperature of which is between 85° C. and 95° C.), is less than that ofa container obtained by a process with ignition. For a liquidtemperature consisting of 90° C. for example, the retraction rate of thecontainer is 2%. Moreover, for a liquid temperature of 95° C., fillingis impossible unless container 2 is deformed (the container is pumpedlike a barrel) beyond what is commercially permissible.

BIBLIOGRAPHY

-   [1] J. P. Trotignon, J. Verdu, A. Dobraczynski, M. Piperaud,    “Matières plastiques: structure-propriétés, mise en oeuvre,    normalisation [Plastic materials: structure-properties,    implementation, standardization]”, Ed. AFNOR/NATHAN, Paris 1996,    ISBN Nathan 2-09-176572-4-   [2] Y. Marco, “Caractérisation multi-axiale du comportement et de la    microstructure d'un semi-cristallin: application au cas du PET    [Multi-axial characterization of the behavior and of the    microstructure of a semi-crystalline: application in the case of the    PET]”, Doctoral thesis of the Ecole Normale Supérieure de Cachan,    June 2003-   [3] M. Chaouche et F. Chaari, “Cristallisation du poly(éthyléne    téréphtalate) sous élongation: étude in-situ par diffraction de R.X    et polarimétrie optique [Crystallization of poly(-ethylene    terephthalate) under elongation: in-situ study by R.X. diffraction    and optical polarimetry],” in Rheology, Vol. 6, 54-61 (2004)-   [4] M. Chaouche, F. Chaari, J. Doucet, Polymer, 44, 473-479 (2003)    “Crystallization of poly(-ethylene terephthalate) under tensile    strain: crystalline development versus mechanical behavior”, in    Polymer, 44 (2003), 473-479-   [5] A. Mahendrasingam, D. J. Blundell, C. Martin, W. Fuller, D. H.    MacKerron, J. L. Harvie, R. J. Oldman, R. C. Rieke, “Influence of    temperature and chain orientation on the crystallization of    poly(ethylene terelphtalate) during fast drawing,” in Polymer, 41    (2000), 7803-7814-   [6] U.S. Pat. No. 6,767,197 (Schmalbach-Lubeca AG)-   [7] French patent FR 2 649 035 (Sidel) and its American equivalent    U.S. Pat. No. 5,145,632-   [8] U.S. Pat. No. 4,512,948 (Owens Illinois, Inc.)-   [9] U.S. Pat. No. 4,476,197 (Owens Illinois, Inc.)-   [10] French patent FR 2 595 EV34 (Sidel) and its American equivalent    U.S. Pat. No. 4,836,971-   [11] European patent EP 1 305 EV18 (Amcor)

1. Manufacturing process of a container (2) in a mold (5) having acavity (8) defining the final shape of the container (2), from a polymerblank (3) heated in advance, this process comprising the followingoperations: introduction of the blank (3) into the mold (5) heated to apredetermined temperature, introduction into the blank (3) of apressurized explosive gaseous mixture, ignition of the gaseous mixture.2. Process according to claim 1, comprising: a pre-blowing operationduring the course of which the blank (3) is stretched, while introducingthe explosive gaseous mixture into it; an ignition operation consistentwith igniting the explosive gaseous mixture; a degassing operationduring the course of which the inside of the container (2) is brought toa pressure close to atmospheric pressure.
 3. Process according to claim1, comprising: a pre-blowing operation during the course of which theblank (3) is stretched, while introducing the explosive gaseous mixtureinto it; an ignition operation consistent with igniting the explosivegaseous mixture; a blowing operation consistent with introducing highpressure air into the container (2); a degassing operation during thecourse of which the inside of the container (2) is brought to a pressureclose to the atmospheric pressure.
 4. Process according to claim 2,comprising, following the ignition operation, a stabilization operation,during which the residual gas deriving from the ignition is maintainedin the formed container (2).
 5. Process according to claim 2,comprising, prior to the degassing operation, a sweeping operationduring the course of which air is circulated in the container (2). 6.Process according to claim 1, comprising: a pre-blowing operation,during the course of which the blank (3) is stretched, while introducingthe explosive gaseous mixture into it; a primary ignition operationconsistent with igniting the explosive gaseous mixture; a primarydegassing operation during the course of which the inside of thecontainer (2) is brought to a pressure close to atmospheric pressure, asecondary pre-blowing operation during the course of which an explosivegaseous mixture is introduced into the container (2); a secondaryignition operation consistent with igniting the explosive gaseousmixture; a secondary degassing operation during the course of which theinside of the container (2) is brought to a pressure close to theatmospheric pressure.
 7. Process according to claim 6, comprising,following the secondary ignition operation, a blowing operationconsisting of introducing high pressure air into the container (2). 8.Process according to claim 6, comprising, following each ignitionoperation, a stabilization operation, during which the residual gasderiving from the ignition is maintained in the formed container (2). 9.Process according to claim 6, comprising, prior to the degassingoperation, a sweeping operation during the course of which air iscirculated in the container (2).
 10. Process according to claim 1,comprising: a pre-blowing operation, during the course of which theblank (3) is stretched, while introducing air into it; a blowingoperation consistent with introducing high pressure air into the blank(3); a primary degassing operation during the course of which the insideof the container (2) is brought to a pressure close to the atmosphericpressure, a secondary pre-blowing operation during the course of whichan explosive gaseous mixture is introduced into the container (2); anignition operation consistent with igniting the explosive gaseousmixture; a secondary degassing operation during the course of which theinside of the container (2) is brought to a pressure close to theatmospheric pressure.
 11. Process according to claim 10, comprising,following the ignition operation, a stabilization operation, duringwhich the residual gas deriving from the ignition is maintained in theformed container (2).
 12. Process according to claim 10, comprising,prior to the secondary degassing operation, a sweeping operation duringthe course of which air is circulated in the container (2).
 13. Processaccording to claim 1, in which the mold (5) is heated to a temperaturegreater than or equal to 100° C.
 14. Process according to claim 1, inwhich the mold (5) is heated to a temperature greater than or equal toabout 130° C.
 15. Process according to claim 1, in which the mold (5) isheated to a temperature greater than or equal to about 160° C. 16.Process according to claim 1, in which the explosive gaseous mixturecomprises air and hydrogen.
 17. Process according to claim 16, in whichthe volumetric proportion of hydrogen is about 6%.
 18. Process accordingto claim 1, in which the explosive gaseous mixture is introduced intothe blank (3) or the container (2) at a pressure greater than or equalto 10 bars.
 19. Process according to claim 3, in which the pressure ofthe air introduced into the blank (3) or the container (2) at the timeof the blowing operation is greater than or equal to 30 bars. 20.Machine (1) for the manufacture of containers (2) from polymer blanks(3), heated in advance, comprising: a mold (5) having a cavity (8)defining the final form of the containers (2), the means (39, 40, 41) ofheating the mold (5); the means (18-24, EV2) of injection of apressurized explosive gaseous mixture into the mold (5), the means (35,36) of ignition of the gaseous mixture.
 21. Machine according to claim20, in which the means (35, 36) of ignition comprise a spark plug (36).22. Machine according to claim 21, in which the spark plug (36) leads toa nozzle (16), suitable for communicating with the inside of thecontainer (2) in order to introduce a gaseous mixture into it. 23.Machine (1) according to claim 21, in which the spark plug (36) leadsinto a stretch rod (12).